In German Patent Specification No. 3 316 838 there is disclosed a method of producing a foam from a plastics material. The plastics material is processed in a tandem extruder system. In the first extruder, the material is fused and mixed with a propellant which is injected into the mixture. This first extruder is a single-screw extruder. Subsequently, the mixture of plastics material and propellant is conveyed into a second extruder wherein it is cooled to just below the reaction temperature of the propellant. The mixture is then extruded to form a tubular film.
It has become increasingly more important for foamed plastics materials to have specific properties. The properties include, inter alia, flame-resistancy, high resistance to acids, and resistance to high pressures, temperatures and/or tensions.
The production of such foamed plastics materials, based on high-quality plastics materials, presupposes that the products are capable of being produced in cellular or foamed form. This prerequisite is realised by the addition of supplementary materials which are suitable for producing foaming in extrusion systems because of their structure and properties.
It has, however, been found that it is extremely difficult to mix a high-molecular weight or high-temperature-resistant plastics material (macromolecule) and a low-molecular weight plastics material, or even a plastics material which had been rendered flame-resistant by additives, with a conventional plastics material and to create a homogeneous product therefrom.
On the other hand, it is commercially necessary to blend very expensive special plastics materials, such as, for example, PPO (polyphenylene oxide), with cheaper low molecular weight conventional plastics materials in order to reduce costs.
An additional problem arises when it is necessary to mix the plastics material homogeneously with a propellant. Since the propellants usually employed, such as fluorochlorohydrocarbons, aliphatic hydrocarbons or azodicarbonamide have properties which differ considerably from the plastics materials to be processed, homogeneous mixing is very difficult. In particular, the propellants can only be processed within narrow temperature and pressure ranges. A lack of homogeneity in respect of the mixture of plastics material and propellant results, which leads to inconsistent foaming. Thus, for example, different thicknesses and cell sizes are produced in a foamed film, or weak spots are formed in other products.